Saturday, April 26, 2008

Are Skeptics Right? Is the World Cooling?

This is one of those moments when one must suck it up and take a hard look at one's convictions. The data have been showing right along that greenhouse gases have been driving climate change during the last half-century or so. Nothing else could explain why the global-average temperature was rising while solar activity declined after 1990.

But then the climate gremlins struck. Skeptics believed themselves fully vindicated when temperature data showed the global-average temperature to have declined during the last five years, with a very sharp drop in the last year. Here are two plots that show the striking effect.

Since we are determined to follow the science where it goes, not where we want it to go, this startling development demanded a cold-blooded analysis, not defensive handwaving.

The puzzle gains difficulty when we look at the data for the northern and southern hemispheres separately, as shown here. The northern hemisphere is continuing to warm up while the southern hemisphere has cooled a surprising amount. Before we can proceed, we have to understand why the two hemispheres would be so dissimilar. In particular, the skeptics can't claim to be right unless they can explain why the northern hemisphere is warming.

As we look at the data for the two, we see that they agree only in broad strokes. When we look at them in detail, we see that there are notable differences. For example, the southern hemisphere was warming in the 1960s while the northern hemisphere was cooling.

That doesn't help us much to understand what's going on, though. What does help us is the explanation of la Niña events, as given here together with some surface-temperature data, given here. The data show the effects of la Niña events, in which stronger winds over the South Pacific cause ocean water to turn over, bringing cold water to the surface. That explains the cooler average surface temperature over the southern hemisphere. The average temperature seems to be headed down; what actually is happening is that the last el Niño event (a period of relative calm over the South Pacific) was fairly strong and the current la Niña event is especially strong, putting a kink in the curve. It is not the case that the world is cooling off; rather, warm water is being driven down to lower depths and colder water is being raised to the surface.

So that's where we stand. La Niña events explain why the southern hemisphere is showing a cooler average surface temperature. Only global warming due to greenhouse gases explains why the northern hemisphere is warming.

It seems reasonable that global warming could be causing the stronger and more-frequent la Niña events, but that's a question for experts.

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